1. Field of the Invention
The present invention relates to a piezoelectric micro-blower suitable for transporting a compressible fluid, such as air, for example.
2. Description of the Related Art
Piezoelectric micro-pumps are used as fuel transporting pumps for fuel cells or as coolant transporting pumps for small-sized electronic apparatuses, such as notebook computers. On the other hand, piezoelectric micro-blowers can be used as air blowers for CPUs in place of cooling fans or as air blowers for supplying oxygen necessary for generating fuel cells. Piezoelectric micro-pumps and piezoelectric micro-blowers both use a diaphragm that can be bent by applying a voltage to a piezoelectric element, and are both advantageous in that they have a simple structure and low profile as well as consuming low power.
Generally, when transporting a non-compressible fluid such as a liquid, check valves made of a soft material, such as rubber or resin, are provided at an inlet and an outlet, and the piezoelectric element is driven at a low frequency of about several tens of Hz. However, when using a micro-blower equipped with check valves to transport a compressible fluid such as air, the fluid can hardly be discharged since the amount of displacement of the piezoelectric element is extremely small. Although maximum displacement can be achieved by driving the piezoelectric element near the resonance frequency of the diaphragm (i.e., first-order resonance frequency or third-order resonance frequency), the check valves cannot be slave-driven since the resonance frequency is a high frequency on the order of kHz. Therefore, a piezoelectric micro-blower which does not include a check valve is preferable for transporting a compressible fluid.
Japanese Unexamined Patent Application Publication No. 2006-522896 discloses a gas-flow generator that includes an ultrasonic driver body having a piezoelectric disc attached to a stainless-steel disc, a first stainless-steel film body disposed on the stainless-steel disc, and a second stainless-steel film body attached substantially parallel to the ultrasonic driver body and separated from the ultrasonic driver body by a desired distance. The ultrasonic driver body can be bent by applying a voltage to the piezoelectric disc. The second stainless-steel film body is provided with a hole in the central section thereof.
Air is vibrated through the hole in the second stainless-steel film body. In the compression process, an inertial jet with high directivity is generated from this hole, whereas in the reverse process, an isotropic flow flowing into a hollow section is generated through this hole. Thus, an intensive jet stream is generated in a direction perpendicular to the surface of the film body. Since this gas-flow generator does not have a check valve, the ultrasonic driver body can be driven at a high frequency.
Furthermore, this gas-flow generator can be used together with a double-sided heat sink to dissipate heat from electrical components. Gas flowing along the surface of the second stainless-steel film body having the hole flows inside a passage along the top surface of the heat sink. The jet stream from the film body passes the heat sink by traveling through the center thereof. Subsequently, the jet stream flows through a passage on the bottom surface of the heat sink.
When transporting gas in the above-described manner, it is possible to generate a desired jet stream by driving the ultrasonic driver body near the resonance frequency thereof, but noise generated near an outlet or an inlet is significant. In general, the human ear can hear sound at a frequency of about several tens of Hz to about 20 kHz, but high-frequency sound in the range of about 7 kHz to about 10 kHz in particular is extremely disturbing to the human ear. Since a passage formed in a space between the second stainless-steel film body and the double-sided heat sink in the gas-flow generator disclosed in Japanese Unexamined Patent Application Publication No. 2006-522896 is merely a straight passage, noise (wind noise) generated near the hole undesirably leaks to the outside through the passage.